Allow authentication with json in body, rather than query params

[dmorrow]

What is the issue?

It is challenging to authenticate to an endpoint that expects JSON in the POST body, rather than the a url-encoded string

Any improvements you suggest

Currently, we are trying to subclass HttpAuthorizer to reimplement the authorize() method. However, we need to overwrite this entire method to change a few lines.

In the Swift library, its as simple as building a URLRequest in the requestFor() method. Can it be this easy in Java too ? 😄

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CC @pusher/mobile

[kn100]

Tylers PR seems to have resolved this!